Clarifier cover

ABSTRACT

In the present invention, there is provided a removable cover for retaining off-gases inside a clarifier. The cover comprises trusses extending across the clarifier and a flexible sheet laid across the trusses. Structural angles enclose the clarifier and constitute a kick plate on the walkways around the clarifier for improving the safety of workers. One side of the flexible sheet is permanently affixed to a first wall of the clarifier. A series of pullers are mounted inside the structural angles for pulling and releasably sealing the three other sides of the flexible sheet against the outmost trusses and against a second wall of the clarifier opposite the first wall. Each puller has a cable-deflector bar extending substantially at a same level as a mounting surface of the puller for increasing an angle of downward pull on the flexible sheet. The flexible sheet is rolled-up along the trusses to expose machinery inside the clarifier.

[0001] This is a continuation of U.S. patent application Ser. No.09/598,682 filed on Jun. 21, 2000.

FIELD OF THE INVENTION

[0002] This invention pertains to flexible covers for clarifiers, andmore particularly, the present invention pertains to the sealing of aflexible cover over a clarifier for maintaining the content of theclarifier in a sealed environment.

BACKGROUND OF THE INVENTION

[0003] A typical industrial clarification installation comprises aplurality of reservoirs or holding tanks to be filled with milleffluent. Clarification is normally effected by alternatively agitatingand letting the effluent settle, and lifting floating scum from thesurface of the effluent or scooping sediments at the bottom of thereservoir. These holding tanks are generally equipped with mechanicalequipment that must be accessible for inspection, maintenance or forrepair.

[0004] The clarification process is often accompanied by a fermentingaction and a generation of odorous bio-gases, and/or the release ofvolatile organic carbons. For environmental reasons, these gases must becollected and treated. Therefore, a clarification reservoir, orclarifier, is preferably covered and sealed to contain the gases. Also,a clarifier preferably has a piping system to transport the gases to agas treatment plant.

[0005] When a removable flexible cover is installed over a clarifier,the installation must resist wind forces that tend to flap portions ofthe cover and stretch it laterally. Also, the attachment of a flexiblecover must be sufficiently strong to permit one or more workers to walkon the flexible cover to fix it if the need arises.

[0006] A clarifier having a removable flexible cover mountable thereonmust also provide for a certain level of safety when the cover isremoved. During maintenance activities for example, it is common to findpower tools, welding machines, acetylene torch carts and maintenancewagons along the perimeter of the clarifier. It is therefore importantto prevent workers and equipment from sliding into the reservoir.

[0007] Examples of various systems available for covering a reservoirare described in the following documents:

[0008] U.S. Pat. No. 3,195,310 issued on Jul. 20, 1965 to C. E.Schroeder;

[0009] U.S. Pat. No. 4,324,370 issued on Apr. 13, 1982 to W. Guard etal.;

[0010] German Patent DE 2,938,496 published on Apr. 24, 1980;

[0011] French Patent FR 2,668,195 published in Apr. 24, 1992.

[0012] Although the flexible covers of the prior art deserve undeniablemerits, it is believed that these systems do not meet all therequirements for a flexible clarifier cover which is securely mountable,removable and safe. It is believed that there continues to be a need fora new removable flexible cover which may be positively sealed, which cansupport workers walking thereon and which provides for a safe workingenvironment.

SUMMARY OF THE INVENTION

[0013] In the present invention, however, there is provided a removableclarifier cover which has safety features incorporated therein, which isparticularly efficient for sealing a clarifier in a manner to resistwind stresses and to support workers walking thereon.

[0014] In a first aspect of the present invention, there is provided aremovable cover for retaining off-gases inside a clarifier having first,second, third and fourth walls. The cover comprises a flexible sheetlaid across the clarifier and having first, second, third and fourthmargins. There are also provided means for retaining and sealing thefirst margin to the first wall, and means for pulling and releasablysealing the second, third and fourth margins to the second, third andfourth walls respectively. The means for retaining and sealing the firstmargin to the first wall comprise a first structural angle extendingalong the first wall and having a leg member aligned upwardly. The meansfor pulling and releasably sealing the second, third and fourth marginscomprise spaced-apart pullers mounted inside second, third and fourthstructural angles extending along the second, third and fourth wallsrespectively and each structural angle has a leg member alignedupwardly.

[0015] In this aspect of the present invention the first, second, thirdand fourth walls, and the flexible sheet are usable for containingoff-gases inside the reservoir and the first, second, third and fourthstructural angles constitute kick plates enclosing the clarifier forenhancing the safety of workers working around the clarifier.

[0016] In another aspect of the present invention, the clarifier covercomprises a plurality of spaced-apart trusses mounted across theclarifier and the flexible sheet is laid over the spaced-apart trusses.In this aspect of the invention, the flexible sheet is selectivelyrolled-up over the spaced apart trusses for accessing equipment insidethe reservoir.

[0017] In accordance with another aspect of the present invention, themeans for pulling and releasably sealing the second, third and fourthmargins against the second, third and fourth walls respectively alsocomprise cables affixed to the second, third and fourth margins of theflexible sheet and individually attached to a respective puller. Each ofthe pullers has a cable deflector bar extending substantially at a samelevel as a plane of mounting of the puller. These pullers areadvantageous for pulling the margins of the flexible sheet against themounting surface of each puller for facilitating the sealing of theflexible sheet against the second, third and fourth walls.

[0018] In accordance with yet another feature of the present invention,there is provided a removable cover for retaining off-gases inside aclarifier having first and second opposite side walls, and a pair ofopposite end walls. A plurality of spaced-apart trusses are mountedacross the first and second side walls. The trusses comprise first andsecond outmost trusses each having an arched plated structure attachedthereto, and a series of intermediate trusses disposed between the firstand second outmost trusses. The flexible sheet is laid over thespaced-apart trusses and the outmost trusses. The flexible sheet hasfirst and second longitudinal margins and first and second transversaledges. The clarifier cover further has means for retaining and sealingthe first longitudinal margin to the first side wall; means for pullingand releasably sealing the second longitudinal margin to the second sidewall, and means for pulling the flexible sheet across the trusses andfor releasably sealing the first and second transversal edges to thefirst and second outmost trusses respectively. The means for pulling andreleasably sealing the second longitudinal margin against the secondside wall further comprise a pipe affixed to the second longitudinalmargin, a first series of pullers mounted to the second wall and aresilient pad extending under and adjacent the first series of pullers.Each puller has means for pulling the pipe and the second longitudinalmargin downward against the resilient pad for sealing the secondlongitudinal margin against the second side wall.

[0019] The means for pulling the flexible sheet across the trusses andfor releasably sealing the first and second transversal edges againstthe first and second outmost trusses respectively comprise grommetsalong the first and second transversal edges of the flexible sheet.Cables extend through the grommets. Second and third series of pullersare affixed to the first and second outmost trusses respectively. Thepullers have means for pulling The cables and the first and secondtransversal edges against the first and second outmost trussesrespectively for sealing these transversal edges against the outmosttrusses.

[0020] The first, second and third series of pullers have ratchet-typemechanisms that are easily worked for selectively releasing the flexiblesheet for opening the clarifier and exposing the equipment inside theclarifier.

[0021] Other advantages and novel features of the present invention willbecome apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] A preferred embodiment of this invention and several variantelements are illustrated in the accompanying drawings, in which likenumerals denote like parts throughout the several views, and in which:

[0023]FIG. 1 is a perspective view of a clarification installationshowing a clarifier having a pair of covers according to the preferredembodiment of the present invention mounted thereon end-to-end;

[0024]FIG. 2 is a cross-section view of the clarifier illustrated inFIG. 1 as seen along line 2-2 in FIG. 1;

[0025]FIG. 3 is an enlarged partial view of the attachment side of theclarifier cover taken from the left side of the cross-section view inFIG. 2;

[0026]FIG. 4 is an enlarged partial view of the openable side of theclarifier cover taken from the right side of the cross-section view inFIG. 2;

[0027]FIG. 5 is a partial perspective view of a preferred truss membersupporting the flexible sheet over a clarifier;

[0028]FIG. 6 is a partial perspective view of one comer of the clarifiercover, taken along the openable side thereof;

[0029]FIG. 6A is an enlarged partial view of the pipe extending alongthe openable side of the flexible sheet and illustrating a keyhole-likeslot in this pipe, as seen in detail circle 6A in FIG. 6;

[0030]FIG. 7 is a front, top and side perspective view of the preferredpuller for retaining the clarifier cover in a sealed condition;

[0031]FIG. 8 is an enlarged partial cross-section view of the openableside of the clarifier cover as seen along line 8-8 in FIG. 6;

[0032]FIG. 9 is an enlarged partial cross-section view along an edge ofthe clarifier cover, showing a first preferred sealing arrangement ofthe flexible sheet over the outmost truss of the clarifier cover, asseen along line 9-9 in FIG. 6;

[0033]FIG. 10 is a schematic illustration of a first preferred sealingarrangement, as seen generally along line 10 in FIG. 9, for sealing thetransversal edge of the cover;

[0034]FIG. 11 is an enlarged partial cross-section view along an edge ofthe clarifier cover, showing a second preferred sealing arrangement ofthe flexible sheet over the outmost truss of the clarifier cover, asseen along a similar location as for the view defined by line 9-9 inFIG. 6;

[0035]FIG. 12 is a schematic illustration of the second preferredsealing arrangement, as seen generally along line 12 in FIG. 11;

[0036]FIG. 13 is a plan view of a triangular cover segment for coveringan opening in a circular clarifier;

[0037]FIG. 14 is a cross-section view of the triangular segment, as seenalong line 14-14 in FIG. 13.

[0038]FIG. 15 is a plan view of a non-rectangular clarifier covered by aplurality of clarifier covers according to the present invention;

[0039]FIG. 16 is an elevation view of the non-rectangular clarifiershown in FIG. 15;

[0040]FIG. 17 is a cross-section view of the openable side of aclarifier cover showing a variant puller mounted along the openableside;

[0041]FIG. 18 is a cross-section view of the attachment side of aclarifier cover showing a variant arrangement for sealing the attachmentside of the flexible sheet to the peripheral walkway;

[0042]FIG. 19 is an end view of the openable side of a clarifier covershowing a variant installation for stretching and sealing the flexiblesheet along one of the outmost trusses;

[0043]FIG. 20 is an end view of the attachment side of a clarifier covershowing additional details of the variant installation shown in FIG. 19;

[0044]FIG. 21 is a cross-section view of the outmost truss as seen alongline 21-21 in FIG. 20;

[0045]FIG. 22 is a plan view of a suspended clarifier cover comprisingsome of the novel elements used in the clarifier cover according to thepreferred embodiment of the present invention;

[0046]FIG. 23 is a cross-section view of the suspended cover as seenalong line 23 in FIG. 22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0047] While this invention is susceptible of embodiments in manydifferent forms, there is shown in the drawings and will be described indetails herein a specific embodiment, with the understanding that thepresent disclosure is to be considered as an example of the principlesof the invention and is not intended to limit the invention to theembodiment illustrated and described. A number of variant elements arealso presented herein to illustrate various manners of construction,installation and operation of the present invention as applied tovarious clarifier configurations.

[0048] A clarification installation showing juxtaposed reservoirs 20each being covered by a pair of clarifier covers 22 according to apreferred embodiment of the present invention is illustrated in FIG. 1.In a typical clarification installation, each reservoir or clarifier 20is divided in two or more sections by one or more intermediate walkways24. In a preferred installation, each section is covered by a clarifiercover 22 according to the preferred embodiment of the present inventionand is openable individually. Both covers 22 are symmetrical and jointlyseal the entire reservoir 20. Peripheral walkways 26 are also providedalong the perimeter of each reservoir 20.

[0049] In the preferred installation, a suction pipe 30 and valve 32 areconnected to the intermediate walkway 24, to draw bio-gases from underthe covers 22 and to convey the bio-gases through a main suction pipe 34leading to a bio-gas treatment plant 36.

[0050] Referring now to FIGS. 2-5, the structure of the clarifier cover22 according to the preferred embodiment is illustrated therein. Thereservoir illustrated therein has concrete perimeter walls 40, 42 eachhaving a T-like configuration and forming the peripheral walkways 26.

[0051] The clarifier cover 22 comprises a series of trusses 44 that areremovably anchored to the opposite side walls 40, 42 of the reservoir.The trusses 44 are made of a single arched aluminum pipe having anoutside diameter of between about 1-½ inches (38 mm) to 2 inches (50 mm)for installation in clarifiers having a width of about between 14 to 20feet (4.3 to 6 m). The spacing between adjacent trusses is about 4 to 6feet (1.2 to 1.8 m).

[0052] These trusses 44 are covered by a flexible gas-impermeable fabricsheet 46 which is referred to herein as the flexible sheet. Thispreferred flexible sheet is a fibre-reinforced nylon-based sheet havingstretch resistance properties to resist shrinking or sagging andsufficient tensile strength to support several workers walking thereonbetween the trusses. The flexible sheet is preferably coated withpolyvinyl chloride or polypropylene to resist a variety of toxic fumesand has properties to resist deterioration by ultraviolet sun rays. Thepreferred flexible sheet is pliable and also has fire retardantproperties.

[0053] The clarifier cover 22 has an attachment side 50 to which theflexible sheet 46 is permanently affixed and an openable side 52,opposite the attachment side 50. The flexible sheet 46 is affixed to theattachment side 50 for being clamped down to the peripheral walkway 26under a first structural angle 54 extending along the attachment side50.

[0054] The flexible sheet 46 has a hem 56 along the openable side 52 ofthe clarifier cover, and an aluminum pipe 58 is mounted inside the hem56 and extends the full length of the clarifier cover 22. The pipe 58has an outside diameter of about 2-3 inches (51-76 mm).

[0055] There is provided along the openable side 52, an elongatedresilient pad 60 made of a resilient material such as soft rubber forexample. A series of pullers 70 is installed along the openable side 52,inside a second structural angle 72 attached to the openable side, forpulling the pipe 58 against the resilient pad 60 and for efficientlysealing the flexible sheet 46 to the openable side 52.

[0056] Each truss 44 has right-angled brackets 80, one at each endthereof. Each angle bracket 80 has a slot 82 in its lower verticalsegment and one or more holes 84 in its upper horizontal segment. In thepreferred installation, the rim wall 86 of each reservoir 20 hasprotruding anchor bolts 88 which coincide in size and locations with theplacement of the trusses 44, such that each truss is attachable to, orremovable from a respective anchor bolt 88, by working the bolt and bylifting the truss 44 out of the bolt 88.

[0057] The takeoff pipe 30 is preferably connected to the upper portionof the walkway 24. A valve 32 is also preferably mounted on the take offpipe 30 to isolate one reservoir 20 from the main suction pipe 34, fordoing inspection, maintenance or repair work on that reservoir withoutaffecting the operation of other reservoirs in a same clarificationinstallation.

[0058] Referring back to FIG. 1, the outmost truss of a clarifier cover22 is enclosed on one side thereof by an arched plated structure 90extending to and sealed against the peripheral walkway 26. The oppositeoutmost truss is mounted to the arched plated structure 92 between twoclarifier covers and constitutes, with the arched plated structure 92,the intermediate walkway 24. The arched plated structures 90, 92 mayhave one or more louvers or portholes therein (not shown) to admit airunder the clarifier cover 22, or to allow visual inspection of thereservoir.

[0059] Referring back to FIG. 2 the clarifier cover is openable byrolling the flexible sheet 46 onto the pipe 58 and along the trusses 44as illustrated. The preferred pipe 58 has a square or hexagonal boss 96at each end. One of these bosses 96 is partly illustrated in FIGS. 4 and6. A socket wrench, a hand wheel 98 or similar tool may be mounted tothese bosses 96 for facilitating the winding of the flexible sheet ontothe pipe 58. When a pair of hand wheels 98 are used, one of which isshown in FIG. 2, two workers, one standing on an intermediate walkway 24and the other one standing on the arched plated structure 90 forexample, can roll up the flexible sheet 46 with ease.

[0060] An elongated tray 100 is defined by the first structural angle 54for receiving and holding the flexible sheet 46 in a rolled-up form. Aseries of gussets 102 are welded inside this first structural angle 54for forming a cradle for retaining the rolled-up flexible sheet abovethe anchor bolts 104.

[0061] When inspection or repair work needs to be done inside thereservoir 20, the flexible sheet 46 is easily removed as described andillustrated, and a number of trusses 44 may be removed and stacked sideby side at one end of the reservoir 20 for example, thereby providingunobstructed access to the equipment inside the reservoir.

[0062] Referring now to FIG. 6, there is illustrated therein one comerof the clarifier cover 22 according to the preferred embodiment. Animportant feature of the clarifier cover according to the preferredembodiment is that there are provided kick plates enclosing each cover.The kick plates are made of the first and second structural angles 54and 72 and two arched structural angles 110,112 mounted over the archedplated structures 90 and 92 respectively. As illustrated, the structuralangles are installed with one leg member extending upwardly and theother leg member, referred to hereinafter as the sole portion, laidagainst the surface of the walkways 26 or the plated structures 90, 92.The height of the kick plates, or the dimension of the uprising legmember, is preferably at least about four inches (10 cm).

[0063] The kick plates, or structural angles 54, 72, 110 and 112 areadvantageous for preventing a worker from slipping and falling into thereservoir, or dropping a power tool or maintenance equipment in thereservoir when effecting maintenance work around the reservoir.

[0064] For further increasing the safety of workers working around theclarifier, there are provided handrails 120 mounted along the clarifiercover. These handrails 120 are mounted into a series of sockets 122affixed to the structural angles. The handrails 120 are removable fromthe sockets for doing maintenance work inside the clarifier.

[0065] Another important aspect of the clarifier cover according to thepreferred embodiment is the use of pullers 70 for sealing the flexiblesheet 46 to the outmost trusses 44 and to the openable side 52 of theclarifier cover. The preferred puller 70 has a frame 130, a main shaft132 mounted inside the frame 130. A ratchet wheel 134 is affixed to themain shaft 132 outside the frame. A pawl 136 is also mounted to theoutside of the frame 130 and interferes with the movement of the ratchetwheel 134 for maintaining a tension on a cable wound around the mainshaft 132. The main shaft 132 also has a square or hexagonal stem 138protruding outside the frame 130 on the opposite side of the framerelative to the ratchet wheel 134 and the pawl 136. The square orhexagonal stem 138 has dimensions for engagement with a socket wrenchfor rotation by a socket wrench. The main shaft 132 also has a slot 140there along and a pin 142 extending across the slot for retaining acable 144 to the shaft.

[0066] The preferred puller 70 also has a pair of flanges 146, forattachment to the sole portion 148 of one of the structural angles 72,110, 112, as illustrated in FIGS. 8 and 9 in particular. The pullers 70which are mounted along the openable side 52 of the cover are preferablyheld by anchor bolts 150 extending through the flanges 146, through thesole portion 148 and through the resilient pad 60, for holding the pad60 down against the surface of the walkway 26. When the puller 70 ismounted to the arched plated structure 90 or 92, the puller 70 isretained to the arched plated structure by means of machine bolts 152extending through the flanges 146, through the sole portion 148 of thestructural angle 110 or 112, and through the arched plate structure 90or 92.

[0067] The preferred puller 70 also has a deflector bar 156 mounted tothe frame, below the flanges 146. The deflector bar 156 is particularlyadvantageous for changing the direction of force of the puller forpulling the flexible sheet downward as illustrated and describedhereinafter, to better seal the clarifier cover 22.

[0068] The sealing of the clarifier cover 22 along the openable side 52is effected by pulling the pipe 58 downward against the resilient pad60. This is effected using several pullers mounted along the structuralangle 72, each having a cable 144 and a knob 158 crimped on the end ofthat cable 144. The knobs 158 are inserted into keyhole-like slots 160which are illustrated in FIGS. 6 and 6A.

[0069] The cables 144 are pulled from under the deflector bars 156 andinto the pullers 70, for pulling the pipe 58 downward against theresilient pad 60 and for pulling the longitudinal margin of the flexiblesheet 46 along the trusses 44 and across the reservoir. The pipe 58 ispreferably held to the hem 56 by means of a plurality of fasteners 162extending through the longitudinal margin of the flexible sheet andthrough the wall of the pipe 58. The preferred spacing between thepullers 70 along the openable side 52 is between 24 inches and 72 inches(0.6 to 1.8 m) for common clarifier installations.

[0070] The releasing of the pullers 70 for opening the clarifier coveris done by working the hexagon stem 138 and the pawl 136 as is customarywith common ratchet-type winches.

[0071] The sealing of the clarifier cover 22 along both outmost trusses44 is effected with another series of pullers 70 spaced apart along thearched structural angles 110 and 112 as illustrated in FIGS. 6 and 9.The flexible sheet 46 has grommets 170 along each transversal edgethereof and a cable loop 172 extending through each grommet. Thelocations of the grommets 170 correspond to the locations of the pullers70 along the structural angles 110, 112. Each cable loop 172 is engagedunder the deflector bar 156 and into the main shaft 132 of each puller70. As illustrated in FIG. 9, the deflector bar 156 is generally closerto the mounting structure 90 as is the upper segment 174 of each truss44. Hence, the edge of the flexible sheet 46 is pulled downward by eachcable loop 172, as indicated by arrow 176 in FIGS. 9 and 10. In thefirst preferred arrangement, the cable loops 172 are parts of acontinuous cable 178 extending along the transversal edge of theflexible sheet 46 and through all grommets 170. The cable 178 hasstraight segments 180 extending between adjacent loops 172. Therefore,when one loop 172 is pulled into a puller 70, the tension of the cableis applied to the respective grommet 170 in a downward direction 176 asindicated previously, and also in a circumferential direction asindicated by arrows 182.

[0072] These compound forces 176, 182 are advantageous for pulling theflexible sheet 46 against the trusses 44 and for preventing the flexiblesheet from slipping across the trusses 44 in conditions of strong windfor example, or when one or more workers walk over the flexible sheetfor any reason. The circumferential tension 182 of the cable 178 causesthe arc defined by the straight segments 180 to have a smaller radiusthan the radius of the upper segment 174 of the outmost truss 44. Thisfeature is advantageous for preventing the slipping of the flexiblesheet across and over the outmost truss 44 in a variety of stressconditions.

[0073] Referring now to FIGS. 11 and 12, there is illustrated therein asecond preferred arrangement for sealing the edges of the flexible sheet46 against the outmost truss 44. In this second preferred arrangement,the kick plate along the arched plated structures 90 or 92 is made of acurved flat bar 190 affixed to a curved structural channel 192, theflanges of which face downward. The web portion of the channel 192 has aseries of slots 194 therein, the locations and spacing of whichcorrespond to the locations and spacing of the grommets 196 along thetransversal edge of the flexible sheet 46. The flexible sheet is lashedto the channel 192 by means of a cable 198 extending inside the channeland through the slots 194 and grommets 196.

[0074] The preferred cable 198 has a series of preformed loops 200 therealong. Each loop 200 is defined by a clip 202. The length ‘L’ of eachloop 200 is a same as a spacing ‘L’ between two adjacent grommets 196.This characteristic is advantageous for lashing the transversal edge ofthe flexible sheet to the channel 192 by inserting each loop through agrommet and into an adjacent loop, as illustrated in FIG. 12. In thissecond preferred installation, both ends of the cable 198 are attachedto a respective puller 70 one of which is shown in FIG. 12. Upontightening of the puller 70, the cable 198 is stretched around thecircumference of the arched plated structure 90 or 92 as illustrated byarrows 204 to further seal the transversal edge of the flexible sheetagainst the outmost truss 44.

[0075] While a rectangular clarifier cover has been illustrated anddescribed, it will be appreciated that the principles and structuralarrangements of the present invention are not limited to rectangularclarifiers, but are also applicable to clarifiers having circular orpolygonal perimeters.

[0076] For example, FIGS. 13 and 14 illustrate partial details of atriangular cover segment 210 for covering an opening in a circularclarifier. The cover segment 210 consists of a flexible gas-impermeablefabric sheet with grommets along the sides thereof. The circularclarifier of interest herein has a rigid cover with one or moretriangular openings, through which inspection and repair work inside theclarifier is carried out. The circular clarifier structure is notillustrated in details herein for being apparent from the previousdisclosure. In this circular clarifier, each triangular opening iscover-able by a cover segment 210, and is openable by rolling the coversegment 210 on a flexible pipe 212, in a similar manner as previouslydescribed for the rectangular clarifier. The cover is openable entirelyas previously described for the rectangular clarifier, or partly asillustrated herein with portions of the cover segment 210 being securelyaffixed to the clarifier structure.

[0077] The cover segment 210 is partly of entirely held against theframe 214 of the clarifier by several pullers 70 mounted insidestructural angles 216 also as previously described and aligned along oneor both sides of the triangular opening. The angles 216 are attached tothe rigid portions of the clarifier cover (not shown), and constitutekick plates along both sides of the triangular opening.

[0078] In this particular installation, the flexible pipe 212 ispreferably made of several straight sections linked to each other byflexible torque transmitting joints 218. The flexible pipe 212 isthereby workable between a straight mode for rolling the cover segmentthereon, and a curved mode to better seal the cover segment 210 againsta circular clarifier wall 220. Although the preferred flexible pipe 212is illustrated herein with joints 218, it will be appreciated that itmay be made of a single section of flexible plastic material forexample.

[0079] Another series of pullers 70 is mounted to the outside surface ofthe clarifier wall 220 and is used to retain the flexible pipe 212 andthe cover segment 210 in a closed mode. The structural angle 222retaining the pullers 70 to the clarifier wall constitutes a guard railfor preventing damage to the cover segment 210 and to the flexible pipe212 by equipment such as lawn mowers moving near the clarifier wall.

[0080] When the triangular cover segment is partly openable, the fixedportion 224 is permanently anchored to the clarifier structure by meansof clamp bars 226 for example along both sides thereof. An intermediarytubular bar 228 is preferably affixed to the fixed portion 224 and heldtight by two pullers 70 mounted to the clarifier structure 214.

[0081] For convenience, the triangular cover 210 segment may also beretained to a channel and flat bar structure 230, such as describedherein as the second preferred arrangement and illustrated in FIGS. 11and 12. In this case, the edge of the triangular segment is affixed tothe channel and flat bar structure 230 by a lashed rope 232 aspreviously described and illustrated.

[0082] Another example of a clarifier having a non-rectangular shape ispresented in FIGS. 15 and 16. In this arrangement, the clarifier 250 hasa octagonal shape and is covered by two rectangular covers 252 and fourtrapezoidal covers 254 separated by walkways 256, one of which 256′extends generally along a diameter of the clarifier. Each of the covers252, 254 has an openable side adjoining the diametrical walkway 256′.Each of the covers 252, 254 is opened by rolling the flexible sheet 260toward the outside of the clarifier in a same manner as previouslydescribed. Similarly, the edges of the flexible sheets are sealed alongthe attachment sides and against the upper segments of the outmosttrusses of each cover in a similar manner as previously described.

[0083] It should be noted that the trusses 262 of the trapezoidal covers254 have a same profile but different lengths. Each of these trusses 262has a low end along the diametrical walkway 256′ and a high end alongone of the outside walls 264 of the clarifier. The high ends of thesetrusses define a series of juxtaposed vertical trapezoidal planes whichare covered and sealed by end plates 266. Each of the outmost trusses isalso covered and sealed by end plates 268.

[0084] The openable side 52 of each cover is sealed against thediametrical walkway 256′ using the resilient pad 60 and pullers 70 aspreviously described, or using alternate pullers as will be describedhereinafter.

[0085] It will be appreciated that the pullers 70 as illustrated in thepreferred embodiment of the present invention are advantageous but notessential to the proper operation of the cover. The illustration in FIG.17 presents an alternate puller arrangement 270 which is also effectivein sealing the flexible sheet 46 to the peripheral walkway 26. In thisalternate puller arrangement 270, a hook member 272 is mounted on athreaded stem 274 anchored to the walkway 26. The hook member 272 isadjustable vertically along the threaded stem 274 by means of a nut 276and washer. The hook member 272 engages with the aluminum pipe 58through an opening (not shown) in the hem 56. The nut 276 is tightenedto pull the hook member 276 and the aluminum pipe 58 downward to sealthe flexible sheet 46 against the structural angle 278.

[0086] When the structural angle 278 consists several short segmentseach anchoring a single truss 44, a continuous S-shaped sealing plate280 is preferably attached to the structural angle segments and made toextend along the entire length of the openable side 52 This S-shapedsealing plate 280 has a lower curved lip 282 sealed against the surfaceof the walkway 26, and an upper curved lip 284 covering the upper edgesof the structural angle segments 278. This upper curved lip 284constitutes the sealing edge against which the openable margin of theflexible sheet 46 is pulled tight by the hook members 272.

[0087] When the structural angle 278 consists several short segmentseach anchoring a single truss 44, another continuous S-shaped sealingplate 280 is also used along the attachment side 50 of the cover asillustrated in FIG. 18. The sealing plate 280 is attached to thestructural angle segments 278 by bolts 288. In this arrangement, theattachment margin of the flexible sheet 46 is affixed to the walkway 26under a clamp bar 290 bolted to the walkway by anchor bolts 292. TheseS-shaped sealing plates 280 constitute kick plates along both sides ofthe clarifier for enhancing the security of workers around theclarifier.

[0088] Referring now to FIGS. 19-21 a variant installation forstretching and sealing the flexible sheet against one of the outmosttrusses is illustrated therein. In this variant installation, a rope 300is threaded into a hem 302 along the lateral margin of the flexiblesheet 46 and is kept taut by a puller 70 affixed to the structural angle278, with the deflector bar 156 thereof set below the top surface of thetruss 44.

[0089] On the attachment side 50 of the cover, the rope 300 is guidedunder a peg 304 affixed to the end plate 306 on the outmost truss. Thepeg 304 is substantially a same distance from the upper segment of thetruss 44 as is the deflector bar 156 on the puller 70. The rope 300 isanchored to the structural angle 278 on the attachment side 50 of thecover.

[0090] The rope is pulled tight by the puller 70, to retain the hem 302at a radius which is smaller than the radius of the upper segment of thetruss 44, whereby the hem 302 is effectively retained against movementacross the truss 44.

[0091] In this variant installation, one or more clips 308 are affixedto the end plate 306 along the arched segment defined by the hem 302,and the rope 300 is inserted under each clip 308. It has been found afew clips along the rope are very effective in preventing any slippageof the flexible sheet 46 over the outmost truss 44, and for enhancingthe safety of workers who may have to walk over the cover.

[0092] Referring particularly to FIG. 20, the clamp bar 290 on theattachment side 50 of the cover may be replaced by a structural angle310 to obtain substantially the same function, while providing anadditional kick plate along the cover.

[0093] In FIGS. 22 and 23, there is illustrated a suspended rectangularcover comprising a sheet of gas-impermeable, flexible, high strengthfabric such as the aforesaid flexible sheet 46 which is stretched acrossand affixed to all four sides of a reservoir 320 using clamp bars 290 aspreviously described herein. This type of cover is more economical andversatile than those already described. These covers are typically usedto capture odorous vapours from the reservoir contents, to blocksunlight thus reducing algae growth or chlorine loss, to protect againstdebris, and to reduce heat loss in the reservoir contents. In this typeof installation, there is no truss or other cross member to supportinwardly-directed reaction loads on the walls of the reservoir. Thewalls of these reservoirs are susceptible of being damaged byinwardly-directed point loadings.

[0094] In the suspended cover described herein, however, the flexiblesheet 46 may be floating at the surface of the reservoir or suspendedloosely from its edges. The flexible sheet is not tensioned across thereservoir opening like a trampoline. It is designed to deflect or sagloosely in the middle. This deflection allows the cover to support snowloads while keeping the fabric tension within safe working limits of thematerial. One or more weight lines 322 are mounted over the cover tocause one or more depressions in the cover surface to collect rainwateror melting snow. One or more drain holes 324 extend through the flexiblesheet 46, along the weight lines to automatically evacuate rain andsnow-melt water from the surface of the cover into the reservoir.

[0095] The weight line 322 also tensions the flexible sheet 46 to reducewind flutter and to prolong the life of the cover fabric. The weightline 322 is secured in its position on the cover by ropes or strapswhich are tied to anchors at the cover perimeter. The weight line 322can be removed and reinstalled by operators without standing on thecover.

[0096] An array of cables 326 is mounted under the flexible sheet 46.Each cable has one end affixed to an anchor 328 on one side or one endof the reservoir 320 and the other end is mounted in a puller 70 orwinch on the other side or the other end of the reservoir. The winches70 allow operators to vary the cables' lengths and tensions. In normaluse, the cables 326 are set to sag loosely below the flexible sheet 46as illustrated in FIG. 23, such that the cover is not supported on thecables. When the cover needs to be opened, the cables 326 are pulledtight to support the flexible sheet 46. The flexible sheet 46 can thenbe released from its clamp bars 290 and can be partially rolled up fromone side or from one end of the reservoir. The array of cables 326 isalso pulled tight to install a new flexible sheet 46 in a newinstallation, or to replaced an aging cover.

[0097] As soon as the flexible sheet is clamped down to the walls 330 ofthe reservoir, the cables 326 are lengthened and left to hang looselyunder the flexible sheet. The major advantage of this installation isthat in normal conditions, the cables do not apply point loading orconcentrated side load on the walls of the reservoir which couldeventually lead to damage to these walls. The loads applied to the wallsof the reservoir by the flexible sheet are evenly distributed andtherefore not detrimental to the structural integrity of the reservoir.

[0098] Although the above description refers to industrial clarifierinstallations, it will be appreciated that the principles of the presentinvention can be applied to other water and wastewater reservoirs andtanks such as: chlorine contact basins; raw water storage tanks;reclaimed water storage tanks, sand filters, aeration basins; digesters;trickling filters; sludge thickeners; equalization basins; etc. It willalso be appreciated that the feature of one embodiment may be used inanother.

[0099] In view of the above, it will be appreciated that many changesand modifications may be made to the illustrated and describedembodiments without departing from the essence of this invention.Therefore, the above description and illustrations should not beconstrued as limiting the scope of the invention which is defined by theappended claims.

I claim:
 1. In combination, a clarifier and a clarifier cover mountedover said clarifier for retaining off-gases inside said clarifier; saidclarifier having an upper opening defined by edges comprising a firstand second opposite edges; said clarifier cover comprising: a pluralityof trusses extending across said opening between said first and secondopposite edges; a flexible sheet laid over said trusses and having afirst and second opposite margins; means for retaining and sealing saidfirst margin to said first edge; means for pulling and releasablysealing said second margin to said second edge, comprising winch meansaffixed to said second edge.
 2. The combination as claimed in claim 1,wherein said opening is a polygonal opening.
 3. The combination asclaimed in claim 1, further comprising a walkway along said second edge.4. The combination as claimed in claim 3, wherein said winch means ismounted to said walkway.
 5. The combination as claimed in claim 4,further comprising a kick plate along said walkway.
 6. The combinationas claimed in claim 5, wherein said winch means comprises a pullermounted to said kick plate.
 7. The combination as claimed in claim 1,wherein said means for pulling and releasably sealing said second marginto said second edge comprises a hem along said second margin, a pipe insaid hem and a plurality of hooks engaged with said pipe, each of saidhooks being movably mounted to a threaded stem anchored to said secondedge.
 8. The combination as claimed in claim 7, wherein said means forpulling and releasably sealing said second margin to said second edgecomprises a kick plate affixed to said trusses.
 9. The combination asclaimed in claim 8, wherein said kick plate is an S-shaped plate havinga lower curved lip sealed against said second edge and an upper curvedlip in contact with said second margin.
 10. The combination as claimedin claim 1, wherein said trusses comprises a pair of outmost trusses andsaid flexible sheet has a pair of lateral margins joining said first andsecond margins, and said clarifier cover further comprises means forpulling and releasably sealing said lateral margins to said outmosttrusses.
 11. The combination as claimed in claim 10, wherein said meansfor pulling and releasably sealing said lateral margins to said outmosttrusses comprises lateral hems along said lateral margins, a ropeextending in each of said hems, and winch means mounted to said outmosttrusses for tightening said ropes.
 12. The combination as claimed inclaim 11, wherein each of said outmost trusses has an arched shape andeach of said rope defines an arc having a radius smaller than a radiusof said arched shape.
 13. The combination as claimed in claim 12,wherein each of said outmost trusses has a plurality of clips therealong engaging said rope.
 14. The combination as claimed in claim 1,further comprising means for rolling said flexible sheet on said trussesfrom said second margin.
 15. A clarifier cover for retaining off-gasesinside a clarifier having a first and second opposite edges, comprising;a plurality of spaced-apart trusses mounted across said first and secondopposite edges; said trusses comprising first and second outmost trusseseach having an arched plated structure attached thereto; a flexiblesheet laid over said spaced-apart trusses; said flexible sheet havingfirst and second longitudinal margins and first and second lateralmargins; means for retaining and sealing said first longitudinal marginto said first edge; means for pulling and releasably sealing said secondlongitudinal margin to said second edge; and means for pulling saidflexible sheet across said trusses and for releasably sealing said firstand second lateral margins to said first and second outmost trussesrespectively; whereby said first and second edges, said first and secondoutmost trusses and said flexible sheet are usable for containingoff-gases inside a clarifier, and said flexible sheet is selectivelymovable over said trusses for accessing equipment inside said clarifier.16. The clarifier cover as claimed in claim 15 wherein said means forpulling said flexible sheet across said trusses and for releasablysealing said first and second lateral margins to said first and secondoutmost trusses respectively comprises lateral hems along said lateralmargins, a rope extending in each of said hems, and winch means mountedto said outmost trusses for tightening said ropes.
 17. The combinationas claimed in claim 16, wherein each of said rope defines an arc havinga radius smaller than a radius of said arched plated structure.
 18. Aclarifier cover for retaining off-gases inside a clarifier having sidewalls, said clarifier cover comprising: a flexible gas-impermeable sheetaffixed to said side walls and sagging between said side walls; an arrayof cables affixed to said side walls and hanging loosely under saidflexible gas-impermeable sheet; and adjustable tensioning means engagedwith said cables for selectively tensioning said cables and forsupporting said gas-impermeable flexible sheet with said array ofcables.
 19. The clarifier cover as claimed in claim 18, wherein saidtensioning means comprises winches affixed to said side walls.
 20. Theclarifier cover as claimed in claim 18, further comprising a weight linelaid over said flexible sheet.